1. Field of the Invention
This invention relates to an apparatus for controlling the electron beam in an image pick-up tube and, more particularly, is directed to an apparatus for controlling the electron beam in an image pick-up tube of the type using a feedback system to control the current level of the electron beam.
2. Description of the Prior Art
Generally, in a photoconduction-type image pick-up tube, such as a vidicon, a cathode electrode emits an electron beam of substantially constant current which is directed towards a target electrode having a photoconversion layer deposited thereon. The electron beam from the cathode electrode is regulated by a first beam control or beam-forming electrode which sharpens the electron beam, while a second accelerating electrode further sharpens the electron beam and accelerates the beam towards the target cathode. The electron beam from the cathode electrode then scans the target electrode and, depending on the amount of incident light from an object image projected on the target electrode, a video output signal is produced. More particularly, as the electrons from the accelerated beam impinge upon the target electrode, some are absorbed thereby to neutralize positive charges that reside there due to incident light from the object image projected on the target electrode. The varying amounts of electron-absorption produce an output current of corresponding intensity.
However, when the amont of light incident on the image pick-up tube increases, the constant current electron beam may not have sufficient charge to neutralize all of the positive charge stored at the various portions of the target electrode with one scanning of the electron beam. In other words, an electron beam shortage occurs. As a result, because the electron beam cannot neutralize all of the charge on the target electrode, the video output signal does not precisely follow increases in the amount of incident light projected on the target electrode. As the incident light increases, the video output signal produced by the target electrode decreasingly reflects the actual image viewed by the pick-up tube. Further, for object images having a very high white level, because all of the charge on the target electrode is not neutralized, a white after-image results when the pick-up tube or television camera moves to view another object image.
If, in order to compensate for the foregoing problem, the current of the electron beam at the cathode electrode is pre-set to a relatively large value to avoid insufficient beam current for high levels of incident light, the focusing state of the electron beam deteriorates, resulting in poor resolution of the object image to be viewed. This is because the focusing state of the electron beam deteriorates with increasing current levels of the beam.
It has therefore been desirable to control the current level of the electron beam in accordance with varying levels of incident light projected on the target electrode. In one device known in the art, a voltage corresponding to the video output signal is applied to the beam control electrode of the image pick-up tube to increase the current level of the electron beam for higher levels of incident light projected on the target electrode. In other words, the current of the electron beam is increased for higher levels of incident light without any corresponding deterioration in resolution. However, it has been found that, in such apparatus, when the voltage applied to the beam control electrode is of a low value, the video output signal is indistinguishable for varying levels of incident light projected on the target electrode. Further, it has been found that the stability of this system is poor, that is, the range within which the current level of the electron beam can be stably controlled is very narrow, and unwanted oscillation results.